For producing low-molecular C2-C4 olefins, in particular propylene, from methanol and/or dimethyl ether, a multitude of processes are known to those skilled in the art, which are usually based on the reaction of an educt mixture containing steam as well as methanol vapor and/or dimethyl ether vapor on a form-selective zeolite catalyst.
DE 100 27 159 A1 describes, for instance, a process for producing propylene from methanol, in which first of all a vapor mixture containing dimethyl ether is produced from methanol vapor on a first catalyst, before said mixture is mixed with steam and is reacted in at least two series-connected shaft reactors with catalyst beds of form-selective zeolite to obtain a product mixture containing propylene. Subsequently, the product mixture is processed in a separating device comprising a plurality of distillation columns, so that there is obtained a fraction rich in propylene with a propylene content of at least 95 vol-%, a fraction containing low-molecular hydrocarbons, which is recirculated to the catalyst beds, and a fraction rich in gasoline hydrocarbons, which is removed from the process. What is, however, disadvantageous in this process is the low yield of propylene, based on the total carbon content of the educt mixture, which among other things is due to the fact that the fraction rich in gasoline hydrocarbons is removed from the process unused.
From EP 0 882 692 B1 there is known a process for producing C2-C3 olefins, in which a mixture of steam and methanol vapor and/or dimethyl ether vapor is reacted in a tubular reactor containing a zeolite catalyst at a temperature between 280 and 570° C. and a pressure between 0.1 and 0.9 bar to obtain a product mixture rich in olefins, which subsequently is separated in a separating device to obtain a C2-C4 olefin fraction with a propylene content of at least 40 wt-%, an aqueous fraction, a gaseous fraction, and a fraction containing C5+ gasoline hydrocarbons. While the three first-mentioned fractions are withdrawn from the process, the product stream containing the C5+ gasoline hydrocarbons is mixed with water, heated in a heater to a temperature of 380 to 700° C., and reacted to obtain C2-C4 olefins in a second reactor containing a zeolite catalyst, before the reaction products are recirculated to the separating device. The yields of C2-C3 olefins obtained with this process, although higher than in the process known from DE 100 27 159 A1, likewise are in need of improvement. In addition, this process is characterized by high costs, not least because of the isothermal procedure as well as the necessary vacuum operation in the tubular reactor.
In the known process, a liquid hydrocarbon product is obtained at the reactor outlet after the condensation apart from the gas mixture rich in propylene, which consists of olefins, paraffins, naphthenes and aromatics. To increase the yield, it would be conceivable in principle to recirculate this liquid product to the reactor, in order to selectively convert the olefins and naphthenes obtained into propylene. The aromatics, however, which likewise were recirculated necessarily and constitute the major part of the liquid product, react with the methanol supplied to the reactor as feed, for instance by alkylation of benzene to obtain toluene, of toluene to obtain xylenes, etc. Since as a result less methanol is available for the selective conversion of propylene, the achievable yield is reduced.
In the prior art, there are known various processes for separating aromatics (benzene, toluene, and xylenes) from hydrocarbon streams. While the liquid-liquid extraction for a long time has been the preferred process for recovering aromatics, an extractive distillation has been proposed quite recently, by means of which mixtures can be separated, whose components have only slightly different boiling points. Special solvents are used to increase the difference in volatility between the components to be separated. The extracting agent and the less volatile component flow to the bottom of the distillation column, where the extracted component is recovered by means of a further distillation. The non-extracted component is removed by distillation at the upper end of the extractive distillation column. For reducing sulfur in fuels, this GT DeSulf process has been presented for instance on the ERTC 7th Annual Meeting, Paris, France, Nov. 18 to 20, 2002, by Lucia Cretoiu, Joseph C. Gentry, Sam Kumar and Randi Wright-Wytcherley “Sulfur Reduction With No Octane Loss—GT DeSulf” or on the 2003 AlChE Spring Meeting, New Orleans, USA, Apr. 1 to 2, 2003, by Joseph Gentry, Sam Kumar and Randi Wright-Wytcherley “Extractive Distillation Applied”.
For reducing the benzene content of fuels, there was in addition proposed the separation by means of high-performance pervaporation membranes (cf. “Benzolgehalt in Kraftstoffen reduzieren”, CIT plus 10/2004, page 45) or “Pervaporation shows promise for separating benzene from aliphatics”, Chemical Engineering 9/2004).